Mechanical and chemical-mechanical planarizing processes (collectively "CMP") are used in the manufacture of microelectronic devices to form a flat surface on semiconductor wafers, field emission displays, and many other microelectronic substrates. FIG. 1 schematically illustrates a planarizing machine 10 with a table or platen 20, a carrier assembly 30 above the platen 20, a polishing pad 21 positioned on the platen 20, and a planarizing fluid 23 on the polishing pad 21. The planarizing machine 10 may also have an under-pad 25 attached to an upper surface 22 of the platen 20 for supporting the polishing pad 21. In many planarizing machines, a platen drive assembly 26 rotates (arrow A) and/or reciprocates (arrow B) the platen 20 to move the polishing pad 21 during planarization.
The carrier assembly 30 controls and protects a substrate 80 during planarization. The carrier assembly 30 typically has a substrate holder 32 with a pad 34 that holds the substrate 80 via suction. A carrier drive assembly 36 typically translates (arrow C) and/or rotates (arrow D) the substrate holder 32. Alternatively, the substrate holder 32 may be a weighted, free-floating disk (not shown) that slides over the polishing pad 21. The combination of the polishing pad 21 and the planarizing fluid 23 generally defines a planarizing medium 28 that mechanically and/or chemically-mechanically removes material from the surface of the substrate 80. The polishing pad 21 may be a conventional polishing pad composed of a polymeric material (e.g., polyurethane) without abrasive particles, or it may be an abrasive polishing pad with abrasive particles fixedly bonded to a suspension material. In a typical application, the planarizing fluid 23 may be a CMP slurry with abrasive particles and chemicals for use with a conventional non-abrasive polishing pad. In other applications, the planarizing fluid 23 may be a chemical solution without abrasive particles for use with an abrasive polishing pad.
To planarize the substrate 80 with the planarizing machine 10, the carrier assembly 30 presses the substrate 80 against a planarizing surface 24 of the polishing pad 21 in the presence of the planarizing fluid 23. The platen 20 and/or the substrate holder 32 move relative to one another to translate the substrate 80 across the planarizing surface 24. As a result, the abrasive particles and/or the chemicals in the planarizing medium 28 remove material from the surface of the substrate 80.
CMP processes must consistently and accurately produce a uniform planar surface on the substrate to enable precise fabrication of circuits and photo-patterns. Prior to being planarized, many substrates have large "step heights" that create a highly topographic surface across the substrate. Yet, as the density of integrated circuits increases, it is necessary to have a planar substrate surface at several stages of substrate processing because non-uniform substrate surfaces significantly increase the difficulty of forming sub-micron features or photo-patterns to within the tolerance of approximately 0.1 microns. Thus, CMP processes must typically transform a highly topographical substrate surface into a highly uniform, planar substrate surface (e.g., a "blanket surface").
In one conventional apparatus for planarizing microelectronic substrates, the polishing pad 21 includes a relatively soft polyurethane material. For example, the polishing pad 21 can be a model number IC1000, manufactured by Rodel, Inc. of Newark, Del., and described in U.S. Pat. No. 5,489,233 to Cook et al. The polishing pad 21 can include surface features to increase the polishing rate, as described in Cook et al. and U.S. Pat. No. 5,177,908 to Tuttle. One drawback with the polishing pads described above is that they may tend to conform to the surface of the substrate 80 and may therefore not planarize the substrate surface uniformly. One approach to addressing this drawback is to increase the hardness and elastic modulus of the polishing pad. For example, model number OXP3000 polyurethane polishing pads, having a hardness and elastic modulus greater than the corresponding hardness and elastic modulus of the IC1000 polishing pad, are available from Rodel, Inc.
In another conventional apparatus for planarizing substrates, the planarizing liquid 23 used with relatively soft polishing pads can include a suspension of abrasive fumed silica aggregates 27, such as are shown in FIG. 2. For example, model number ILD1300 planarizing liquids having a suspension of fumed silica aggregates 27 such as those shown in FIG. 2, are available from Rodel, Inc. The fumed silica aggregates 27 can be formed by reacting SiCl.sub.4 and/or SiH.sub.x Cl.sub.y with oxygen in a burning process to form SiO.sub.2 particles. As the SiO.sub.2 particles cool, they collide and adhere to each other, forming the three-dimensional aggregates 27 having a fractal configuration and a relatively large surface area.
One problem with the fumed silica aggregates 27 is that they can scratch or otherwise damage the substrate 80 as a result of their rough, three-dimensional shapes. One approach for addressing this problem has been to form abrasive particles having less surface area and less roughness than the silica aggregates 27. For example, planarizing liquids having spherical abrasive particles are available from Rodel, Inc. under the trade name Klebosol.
One problem with the planarizing solutions having spherical abrasive particles occurs when they are used with relatively soft polishing pads and/or with polishing pads having a porous planarizing surface. The combination of relatively soft polishing pads and planarizing liquids with spherical particles may not uniformly planarize the surfaces of microelectronic substrates because the polishing pads may conform to the surface of the substrate, as discussed above. The porous polishing pad may not planarize the substrate at an acceptable rate because the pores reduce the surface area of the polishing pad that contacts the substrate.